This invention relates generally to carbon-carbon substrates and methods for producing parts using the substrates, and in particular, to a filamentized composite fiber substrate and method for producing a carbon-carbon part using the substrate.
A brake disc for an aircraft or an automobile requires a material having high heat resistance and long wear. For some applications, asbestos is used due to its heat resistance properties. In addition to asbestos, carbon may also be used, although conventional carbon-carbon brake products are expensive and historically restricted to aerospace or automotive racing applications.
Generally, a substrate of carbon fiber or carbon precursor may be used to produce a conventional carbon-carbon part with sufficiently high heat resistance values for use in, for example, an aircraft braking system. These conventional parts require a complicated time consuming process to produce a part with sufficient carbon to provide the necessary high temperature characteristics. These conventional carbon-carbon parts are expensive due to the complicated manufacturing process. There are a number of different types of substrates used to make conventional carbon-carbon parts including discontinuous carbon fiber molding compound, non-woven air lay carbon fiber substrates, woven carbon fiber substrates, or braided carbon fiber substrates.
To produce a conventional carbon-carbon part from a carbon fiber substrate that may be used, for example, for an aircraft brake disc, a plurality of carbon fiber substrates are available. These substrates may be stacked on top of each other to a desired thickness and then the stacked substrates may be needle-punched together, as is known in the art, to join or consolidate the substrates to each other by intermingling carbon fibers between the layers of substrates. This consolidation of the substrates creates a preform. The preform may then be batch carbonized, in which the preform is placed in an oven at 800 to 1100 degrees Celsius, to char the fiber of the substrate and increase the carbon content of the preform. Next, due to shrinking caused by the carbonization, the carbonized preform may be die cut to obtain the desired preform shape. These preforms may then have additional carbon atoms deposited on the carbon fibers of the preforms by using a chemical vapor deposition (CVD) process. In the CVD process, the preform is placed in an evacuated chamber and a carbon bearing gas, such as methane, is introduced into the chamber which when subjected to temperature releases carbon atoms that settle/infiltrate into the preform. The CVD process may increase the carbon content and density of the preform. The preform may then be heat treated to reorient the carbon atoms to a more energetically favorable configuration, machined if necessary, and treated with an anti-oxidant to form the finished carbon-carbon part.
The conventional preform process, as described above, and the conventional carbon-carbon parts have several problems. First, the batch carbonization process is slow and time consuming, taking hours or days which increases the cost of the part. Second, the batch carbonized preforms made from conventional substrates have a limited amount of carbon fiber surface area available so that fewer carbon atoms generated during the CVD process are able to settle/infiltrate into the preform. The lower level of carbon atom pick-up during the CVD process may require that the preforms undergo additional CVD processing and surface grinding steps to achieve the desired density. Third, it is difficult due to the nature of the process to add chemical or material additives to the preforms for the enhancement of performance characteristics because the additives may only be added to the preform after the consolidation step. Fourth, any material removed from the preform during the shaping and die cutting processes cannot be reused because there is no method for recycling this scrap material back into the preform manufacturing process. Thus, due to the above four problems carbon-carbon parts produced using the conventional preform process are typically too expensive to use for most commercial applications.
Another conventional substrate uses carbon fibers that are impregnated with a suitable binder and then the impregnated substrate may be compressed under heat and pressure to form the near net shape preform. The preform is then batch carbonized to char the binder via condensation of the binder into carbon. The binder may be liquid furfuryl alcohol polymer catalyzed with maleic anhydride. Once again, this substrate requires a batch carbonization process step in order to char the binder. Still another substrate for a carbon-carbon part uses carbon fibers, that may be oxidized polyacrylonitrile (PAN) fibers that may then be carbonized to form the carbon preform that may be subjected to the chemical vapor deposition (CVD) process. This substrate also requires a carbonization step.
None of these conventional materials for producing carbon-carbon parts permits the elimination of the batch carbonization step, which increases the cost of the final part. In addition, none of the conventional materials provide a sufficient surface area to permit an efficient rate of densification during the CVD process. The conventional materials also do not provide a method for recycling scrap pieces of the substrate for reintroduction into the preform process. As such, conventional carbon-carbon parts are too expensive to be used in most conventional commercial applications.
Thus, there is a need for a composite material substrate and a method for producing carbon-carbon parts using a substrate which avoids these and other problems of the known substrates, processes and carbon-carbon parts, and it is to this end that the present invention is directed.
The invention provides a composite material substrate having primarily filamentized carbon fibers held together by a binder that affords improved processability over a conventional carbon substrate. The cost of the parts produced using the filamentized composite substrate are reduced for several reasons, including that the filaments, primarily carbon, in the substrate have more surface area exposed, so that fewer fibers are required to achieve the same densification levels and less of the substrate is needed for each final part. In addition, any scraps of the substrate generated during the process may be easily recycled back into the substrate manufacturing process. Carbonized carbon fibers used in the filamentized composite substrate permit the elimination of a batch carbonization step currently used with conventional substrates. Due to the substrate having a higher surface area than conventional substrates, a higher percentage of carbon atoms are also attached to the filaments during the chemical vapor deposition (CVD) process, which reduces the time required to produce a part with a desired density. The binder used in the substrate is readily removed during the CVD process so a batch carbonization step is not necessary. The reduced cost may make these carbon-carbon parts manufactured from filamentized composite substrates suitable for conventional commercial applications.
The invention also provides a substrate for a preform that may have a plurality of discontinuous fibers, primarily carbon, held together by a binder compound wherein the binder compound may be readily displaced during the CVD process. Thus, the binder may hold the carbon filaments in place prior to the CVD process and then may evaporate during the CVD process and be replaced by the carbon atoms.
The invention further provides a substrate that may be produced using a paper process in which the fibers, primarily carbon, and the binder may be combined together to form a substrate that may be used to manufacture a carbon-carbon part. The process for producing a filamentized composite substrate may permit additional chemicals or materials to be easily added to the substrate via the paper manufacturing process, to enhance a variety of characteristics of the substrate and the carbon-carbon part made from the substrate, such as its hardness, coefficient of friction, or oxidative resistance. In another embodiment of the invention, a completed filamentized composite substrate may have resin added to the substrate via a prepregging process in order to increase the friction/wear results. This same resin may also be used as a vehicle, such as a high solids solution, to distribute special additives into the filamentized composite substrate to enhance other properties of the substrate and the carbon-carbon part made from the substrate. In another embodiment of the invention, typically the preform for a carbon-carbon part is of a thickness greater than that of the individual filamentized composite substrate sheet, therefore the preform may consist of a plurality of filamentized composite sheet substrates. This plurality would constitute a multilayer construction to achieve the desired preform thickness. Thus, the filamentized composite substrate provides an opportunity to easily tailor the through thickness properties of the preform by altering the properties of individual filamentized composite substrates that are used to manufacture the preform. As an example, the resin content of the inner layer substrates of a preform may be higher than those of the outer layers to increase the density of the preform at its interior, which may result in substantially reduced chemical vapor deposition (CVD) processing time.
Thus, in accordance with the invention, a carbon-carbon part having a filamentized composite substrate is provided having a substrate of a plurality of discontinuous filaments, primarily carbon, and a binder that binds said filaments together to form a substrate, and a plurality of substrates consolidated to form a preform, and a plurality of carbon atoms deposited onto the filaments at a predetermined temperature and pressure such that said binder is removed completely from said filaments and replaced by said carbon atoms to form a dense carbon-carbon part.
A method for producing a carbon-carbon part is also provided, comprising producing a substrate, the substrate comprising a plurality of discontinuous filaments, primarily carbon, and a binder that binds said filaments together to form a substrate, producing a preform by consolidating said substrates, and depositing carbon atoms, by chemical vapor deposition (CVD) at a predetermined temperature and pressure onto the filaments such that said carbon atoms replace said binder within said preform and said carbon atoms density said preform to form a carbon-carbon part.